On 25 August 2003, NASA’s third Great Observatory was launched to explore the secrets of our Universe at infrared wavelengths. Initially known as the Space Infrared Telescope Facility, Spitzer was renamed (as is traditional in NASA missions after they successfully launch) in December of 2003 in honour of American astronomer Lyman S. Spitzer Jr. who, in 1946, wrote a paper entitled “Astronomical Advantages of an Extra-Terrestrial Observatory”, advocating for space-based astronomy. By the 1970s, astronomers were already considering the idea of an infrared space telescope, culminating in the launch of the Infrared Astronomical Satellite (IRAS), which performed the first infrared survey of the sky. The idea of constructing a follow-up, massive infrared observatory gained traction after “The Decade of Discovery in Astronomy and Astrophysics” report, published in 1991, listed this type of telescope as the highest priority in space-based astronomy. This new mission would eventually become the Spitzer Space Telescope.

The scientists and engineers involved in the design and construction of the telescope could have not imagined that it would still be operating today, 15 years later. The initial requirements called for a 2.5-year mission, which ended up extending to 5.5 years, in a period known as the “cold mission” phase, where the instruments on board were kept just a few degrees above absolute zero by liquid helium. After the 360 litres of coolant ran out on 15 May 2009, two of the instruments stopped working (the Infrared Spectrograph and the Multiband Imaging Photometer). However, Spitzer was able to continue operations because two of the four detector channels of its third instrument — the Infrared Array Camera — still functioned at peak performance at a higher temperature of 28K. These two cameras (at 3.6 and 4.5 microns) have driven Spitzer’s exploration since entering into the “warm mission” phase in July 2009.

Over its 15 years of operations, Spitzer has extended our understanding of the Universe on all spatial scales — from our own Solar System, to planets revolving around other stars, to the farthest and earliest galaxies in the Universe. Working in collaboration with other ground- and space-based facilities that detect electromagnetic radiation in complementary wavelengths, Spitzer has helped us gain a comprehensive picture of many celestial phenomena.

The infrared detectors of Spitzer have helped to discover some of the more distant objects in the Universe, such as the galaxy GN-z11 — the farthest galaxy known, at about 13.4 billion light-years — contributing to the understanding of galaxy formation and evolution. In addition, during recent years, the spacecraft has been used in the detection and characterisation of exoplanets — a scientific goal for which it was not originally designed. Regardless, the telescope’s accurate star-targeting has allowed researchers to use tools such as microlensing and the transit technique to study exoplanets. Spitzer also played a key role in the detection of seven Earth-size planets orbiting the TRAPPIST-1 star. Three of those planets were found to lie in the habitable zone of the system, rendering this one of the most remarkable discoveries in history.

The Spitzer Space Telescope has also participated in other major discoveries along its impressive 15-year run. Some of these studies include:

the detection of Buckminsterfullerenes molecules (“buckyballs”) in space;

the discovery of the largest known ring around Saturn;

the creation of one of the most extensive maps of the Milky Way;

Imaging of stellar nurseries;

the identification of large numbers of massive galaxy clusters.

Since 2016, Spitzer has been in what is known as the “Beyond phase”, and it is scheduled to operate until November 2019. After that, NASA’s next space observatories, the James Webb Space Telescope and the Wide Field Infrared Survey Telescope (WFIRST), will keep unravelling the biggest mysteries of the Universe using infrared light and will, undoubtedly, build upon the revolutionary legacy built by Spitzer.

ANDRÉS PLAZAS is a native of Colombia and a Research Scientist at the Astronomical Society of the Pacific. He is part of the Dark Energy Survey and Dark Energy Science Collaboration of the Large Synoptic Survey Telescope, working on gravitational lensing, astronomical instrumentation and education and public outreach.